Most plant parasitic nematodes are less than 2 mm in length and develop from an egg through three or four juvenile stages (J1-J3 or J4) to adult in a life cycle lasting from a few weeks to several months. Ectoparasites and endoparasites occur and more species attack roots than aerial tissues. With very few exceptions, the nematodes use a hollow stylet both to pierce plant cell walls and to withdraw cell contents. Migration in the plant involves intracellular penetration through perforated cell walls or bodily movement between cells. Some endoparasites migrate short distances into plants before feeding whereas others move continually or rely on host growth to assist their distribution within the plant.
Several genera including both the economically important cyst and root-knot nematodes modify plant cells into feeding sites able to support sedentary females. Such individuals grow within a few weeks by up to 1000.times. ensuring a high fecundity.
Cyst nematodes (principally Heterodera and Globodera spp) are key pests of major crops. Heterodera glycines is the principal pathogen of soybean in the USA with an economic effect that may lie between US$500-1000M a year. Heterodera shachtii (Beet cyst nematode) is a major constraint on sugar beet growers in the EC and parts of the USA and Heterodera avenae (cereal cyst nematode) is a cosmopolitan pathogen of cereals with particular importance in more arid soils for instance parts of Australia. Potato cyst nematodes Globodera rostochiens and G. pallida occur in many areas of potato cropping. They are highly damaging causing an estimated .English Pound.10-50M a year loss to the UK potato industry from their direct and indirect effects on production.
Root-knot nematodes (Meloidogyne spp) are associated with tropical and subtropical soils and few other pathogens out rank them in importance to world agriculture. There are many species but five are responsible for the majority of crop damage with M. incognita estimated to account for about 66% of all incidences of economic loss to this genus. Severity of crop losses varies but overall losses of 11-25% have been estimated for a wide range of crops in major geographical regions of the tropics.
Cultural, chemical and resistant control are the chief approaches in current use, often in an integrated manner. There is an urgent need to improve control since nematicides are among the most unacceptable compounds in widespread use. One carbamate, aldicarb and its breakdown products are highly toxic to mammals and have polluted groundwater in the USA and presumably other areas where this pesticide is widely used. Cultural control includes hidden losses that are unacceptable to specialist growers or those with few alternative, economic crops. Resistant cultivars are not always available and are often out yielded by the best susceptible cultivars so again they involve hidden losses unless nematode damage is certain to occur. The inadequacy of current crop protection testifies to the need for an effective approach to the control of nematodes.
Economic densities of cyst nematode characteristically cause stunted plants with a root system occupying a small soil volume. The diseased plants show symptoms of mineral deficiencies in their leaves and wilt readily. Yield losses are related to the severity of parasitism above a tolerance limit and can be substantially above 50% for some species. Root knot nematode causes many of the effects described for cyst nematodes with the addition that the root system is often heavily galled with increased accessibility to secondary pathogens.
A few nematodes are vectors of a narrow range of plant viruses (NEPO viruses by, Xiphinema, Longidorus, and certain TOBRA viruses by, Trichodorus). For example, Xiphinema spp. transmit the GFLV virus to vines. In addition nematodes in association with fungi are transmitted by specific insect vectors and cause a few important conditions such as pine wilt disease and red ring disease of coconut. In restricted areas of Australia a nematode introduces a Corynebacterium to the seed head of rye grass which then becomes highly toxic to grazing sheep. Disease associations with both bacteria and fungi particularly Fusarium spp contribute considerably to the economic status of Meloidogyne spp. Beneficial nodulation of legumes by Rhizobium spp can also be suppressed by soybean cyst and pea cyst nematodes.
Resistance of crops to nematodes is clearly an important goal. For nematodes, resistance is defined by the success or failure of reproduction on a genotype of a host plant species. Dominant, partially dominant and recessive modes of inheritance occur based on one to three plant genes. A gene-for-gene hypothesis has been proposed in some cases with typically a dominant R-gene for resistance being countered by a recessive V-gene for virulence in the nematode. Two examples of resistance introduced by breeders are as follows.
In relation to Globodera spp., different sources of resistance occur and allow subdivision of populations of potato cyst nematode in Europe into five forms of G. rostochiensis (Ro1-5) and three of G. pallida (Pa1-3). These pathotypes are defined as forms of one species that differ in reproductive success on deemed host plants known to express genes for resistance. The H1 gene conferring resistance to Globodera rostochiensis Ro1 and Ro4 is virtually qualitative and widely used commercially. Within the UK, cv Maris Piper expresses H1 and is a highly successful resistant cultivar. Unfortunately, its widespread use in Britain is correlated with an increased prevalence nationally of G. pallida to which it is fully susceptible.
Secondly, in relation to Meloidogyne spp., morphologically similar forms or races occur with differential abilities to reproduce on host species. The standard test plants are tobacco (cv NC95) and cotton (cv Deltapine) for the 4 races of M. incognita whereas the two races of M. arenaria are differentiated by peanut (cv Horrunner). The single, dominant gene in tobacco cv NC95 confers resistance to M. incognita races 1 and 3 but its cropping in the USA has increased the prevalence of other root-knot nematodes particularly M. arenafla. Most sources of resistance are not effective against more than one species of root-knot nematode with the notable exception of the LMi gene from Lycopersicum peruvanium which confers resistance to many species except M. hapla. Another limitation of resistance genes identified in tomato, bean and sweet potato is a temperature dependence which renders them ineffective where soil temperature exceeds 28.degree. C.
There is clearly still a need for further and better resistance of susceptible and commercially important crops against nematodes. Novel resistance should prove of lasting value since nematodes do not have many generations per growing season and changes in importance of pathotypes arise from selection of pre-existing forms in the field rather than from mutation following introduction of the cultivar. The latter process does not occur readily for nematodes. For instance field resistance to nematicides does not occur in contrast to widespread insecticide resistance in aphids and other insects.